Ultrafiltration (UF) feed water requirements and pre-treatment

Ultrafiltration is used in water treatment and other industrial purification, concentration and separation processes, either as a pre-treatment for the process or as a deeper treatment for the process. In widely used water treatment processes, it is often used as a means of deep purification. According to the characteristics of hollow fibre ultrafiltration membranes, there are certain pre-treatment requirements for water supply. 

This is because suspended matter, colloids, microorganisms and other impurities in the water can attach to the membrane surface and contaminate the membrane. As the ultrafiltration membrane water flux is relatively large, the concentration of retained impurities on the membrane surface increases rapidly to produce the so-called concentration polarisation phenomenon, and more seriously, some very small particles will enter the membrane pores and block the water channel. In addition, microorganisms in the water and their metabolic products generate sticky substances that will also adhere to the membrane surface. All these factors can lead to a reduction in the water permeability of the ultrafiltration membrane and changes in separation performance. There are also limits to the temperature, pH and concentration of the ultrafiltration water supply. Therefore, the ultrafiltration water supply must be properly pretreated and the water quality adjusted to meet the water supply requirements in order to extend the life of the ultrafiltration membrane and reduce the cost of water treatment.

a. Killing of microorganisms (bacteria, algae).

When the water contains microorganisms, after entering the pre-treatment system, some of the intercepted microorganisms may adhere to the pre-treatment system, such as the media surface of the multi-media filter. When adhered to the surface of the ultrafiltration membrane grows and multiplies, it may completely block the micropores or even completely block the inner lumen of the hollow fibres. The presence of microorganisms can be extremely harmful to hollow fibre ultrafiltration membranes. Removal of microorganisms such as bacteria and algae from raw water must be taken seriously.

b. Reducing turbidity of incoming water.

When the water contains suspended matter, colloids, microorganisms and other impurities, will make the water produce a certain degree of turbidity, the turbidity of the light through the material will have an obstructive effect, this optical effect and the number of impurities, size and shape have a relationship. Measure the turbidity of the water is generally expressed in terms of the degree of etching, and the turbidity produced by 1mg/lsio2 for 1 degree, the greater the degree, the more impurities. In different areas of the water supply turbidity has different requirements, for example, for general domestic water, turbidity should not be greater than 5 degrees.

c. Removal of suspended and colloidal substances.

For impurities with a particle size of 5μm or more, 5μm filter precision can be used to remove them, but for microscopic particles and colloids between 0.3 and 5μm, it is difficult to remove them using the conventional filtration techniques mentioned above. Although ultrafiltration has an absolute removal effect on these particles and colloids, the harm to the hollow fibre ultrafiltration membrane is extremely serious. In particular, colloidal particles have an electrical charge and are aggregates of material molecules and ions. Colloids can exist stably in water, mainly as a result of the mutual repulsion of colloidal particles with the same electrical charge. To the original water to add and colloidal particles of the opposite electrical properties of charged substances (flocculants) to break the stability of colloidal particles, so that the charged colloidal particles neutralised into electrically neutral and dispersed colloidal particles coalesced into large clumps, and then the use of filtration or sedimentation can be more easily removed.

d. Removal of soluble organic matter.

Soluble organic matter can not be completely removed by flocculation and sedimentation, multi-media filtration and ultrafiltration. At present, most of the oxidation method or absorption method is used.

(1) Oxidation method The use of chlorine or sodium hypochlorite oxidation, to remove soluble organic matter effect is relatively good, in addition ozone and potassium permanganate is also a good oxidant, but the cost is slightly higher.

(2) Adsorption method The use of activated carbon or macroporous adsorption resin can effectively remove soluble organic matter. However, alcohols and phenols that are difficult to adsorb still need to be treated by the oxidation method.